There are, at present, a variety of so-called "electrochemical" apparatus and processes in which an input of electrical power is employed in order to bring about activity at a working electrode. These electrochemical processes and apparatus are generally employed to treat solutions, such as waste water and plant effluents, in order to reduce the concentration of metal contaminants to levels which are acceptable, particularly in view of the present stringent environmental regulations, and to recover these metal contaminants.
There are two general categories of such electrochemical processes depending on their most significant limiting factor. The first group includes processes whose reaction rates are kinetically controlled, i.e., the reaction rates are limited by the speed of the reactions at a working electrode. In these processes, the solution or electrolyte being treated contains high concentrations of electro-active species. An example of one such process is the electro-refining of zinc, where there is inherently a high concentration of zinc in the electrolyte.
The second group of electrochemical processes includes those in which the reaction rates are controlled by mass transfer considerations, rather than by kinetic requirements, i.e., the reaction rates are limited by how much of the contaminants can be brought into contact with a working electrode in a given time. In contrast to the electrodes used in kinetically controlled processes, the working electrodes used in these mass transfer controlled processes must exhibit characteristics which enhance the obtainable mass transfer rates. One such characteristic is a large surface area to volume ratio. Attempts have been made to achieve acceptable surface area to volume ratios by utilizing packed beds of fibrous or granular material (see, for example, U.S. Pat. Nos. 2,563,903; 3,450,622; 3,457,152; and 3,827,964), as well as active beds which can move in a flow of electrolyte. These attempts have suffered, however, from distinct disadvantages based primarily on the difficulty of providing a uniform and controlled electrical potential throughout the electrode to make full use of the surface area. The use of granular or fibrous beds is also disadvantageous because the electrolyte can channel around the granules or fibers, thereby bypassing the effective portion of the electrode and, consequently, deleteriously affecting the effectiveness of the electrode. Thus, two general disadvantages of the prior art mass transfer controlled processes are low current efficiency and low conversion completeness. As a result of these major drawbacks, none of the prior art mass transfer controlled processes has achieved significant acceptance.
In both the kinetically controlled processes and the mass transfer controlled processes, one of the prime considerations is the method of recovering the electroactive material removed from the electrolyte and deposited on a working electrode. It is generally necessary to conduct a stripping operation to remove the deposited material from the working electrode prior to the subsequent use thereof. The working electrodes used in these processes are sometimes made from the same material that is to be stripped therefrom, so that the resulting product can be used directly. More commonly, however, these electrodes are designed for mechanical stripping. In addition, in other cases, the electrode must meet other requirements, such as those described in U.S. Pat. No. 3,953,312, where the prime consideration is that the electrode be combustible so that silver deposited on the electrode can be recovered by melting during combustion.
More recently electrodes and reactors have been developed which employ carbon fibers in a manner so as to both provide a large surface area to volume ratio and at the same time limit fluctuations in the electrical potential throughout the electrode. Such electrodes and reactors are described, for example, in U.S. Pat. Nos. 4,046,663; 4,046,664; 4,108,754; 4,108,755; and 4,108,757. These electrodes and reactors suffer, however, from the same channeling and bypass problems which plaque the granular or fibrous bed electrodes described above.
Carbon fiber electrodes and reactors therefor have also been proposed, at least on a laboratory scale, by D. Yaniv and M. Ariel in an article appearing in the Journal of Electroanalytical Chemistry, Volume 79 (1977), pages 159 to 167. The structure disclosed in this article includes an electrode of graphite cloth positioned in a frame defining an opening having an area of 2.4 cm.sup.2. The article states that the results obtained confirm the feasibility of exploiting graphite cloth as a practical electrode material suited for flow-through configurations. However, the article goes on to indicate that, although the laboratory reactor worked well, it would be necessary to undertake further work to optimize a reactor using a graphite cloth electrode.
A more recent approach to an electrode for use in mass transfer controlled environments, such as in connection with dilute electrolyte solutions, is disclosed in Japanese Patent No. 67267/76 which was published on June 10, 1976 and assigned to Mitsui Petrochemical Industries Ltd. This patent discloses the use of a porous carbon electrode in connection with an electrode base material which the patent discloses can be any one of a number of well-known electrode materials, such as platinum, iron, copper, nickel, silver, lead and certain alloys thereof. The patent also discloses the use of carbon fibers in various forms, such as cloths, fabrics, felts and carbon fiber papers, to cover a base material in the form of a plate, tube, mesh or plate with holes therein. Furthermore, in Example 1 of this patent, the cathode employed comprises a titanium plate which is plated with platinum and then covered with a layer of carbon fiber fabric. Thus, in effect, a platinum cathode is provided. This patent does not deal with the question of how metals can be recovered from such electrodes so that the concentration of metallic ions can be reduced to extremely low levels in real time in an economical manner.